Most politicians and party leaders fighting to win over Latino voters have probably never heard of labor organizer Eliseo Medina, but they should have.

The international secretary-treasurer of the Service Employees International Union is the engine behind a massive Latino voter mobilization and turnout effort that could prove decisive in 2012.

A Mexican immigrant and former California grape picker, Medina has toiled for more than a decade to boost political engagement among the nation's 50.5 million Latinos. Though turnout among Hispanics still lags behind other voting blocs, Medina's allies say this could be his breakthrough year.

"If Latino turnout is sizable, and if Latino voters are critical, he's going to be the most influential player in American politics that people don't know," said Frank Sharry, executive director of America's Voice, a progressive immigrant reform group that's worked with Medina, who cut his teeth as an activist in the 1960s alongside labor and civil rights leader Cesar Chavez.

The number of Latinos voting will increase to 12.2 million this fall, up from 10 million in 2008, according to projections by the National Association of Latino Elected and Appointed Officials. The likely beneficiary will be President Barack Obama, who leads GOP presidential nominee Mitt Romney among Latino voters by a more than 3-to-1 ratio, according to Latino Decisions.

But while Romney has drawn Latino fire for opposing the DREAM Act and endorsing self-deportation, Obama has disappointed many Latinos by failing to fulfill his immigration reform promises. Obama's June executive order to halt deportation of some young undocumented immigrants won him some points with Latinos. But it's unclear whether that will be enough to boost Latino turnout beyond its customarily low 50 percent.

Medina, 66, insists that his goal isn't to promote one party or candidate over the other but to convince Latinos to become citizens, to register and to vote. This year his top tool is Mi Familia Vota, a 501(c)(4) social welfare group that's working with 150 community-based organizations, including business leaders, civic groups and churches. Mi Familia Vota and its allies have set out to register 650,000 Latino voters. With an affiliated 501(c)(3) charity, Mi Familia Vota will spend as much as $7 million in this election cycle.

"This is strictly nonpartisan," Medina said of Mi Familia Vota, which grew out of a similar campaign launched a dozen years ago to organize Los Angeles workers. "We tell people: Here's the agenda, and it's very clear. Immigration reform, education, health care, good jobs. That's what we're for. Now you look at the candidates and the parties and decide who best will represent those views and vote."

But Medina is second-in-command at the politically influential SEIU, which has endorsed Obama and has spent $12 million in this election - more than any other union, according to the Center for Responsive Politics. That includes a joint $4 million campaign with the leading pro-Obama super PAC, Priorities USA Action, on Spanish-language ads targeting Romney for his immigration positions.

"We think this is the most important election of our lifetime and that Latinos have the opportunity to shape the national agenda moving forward," said Mary Kay Henry, president of the SEIU, whose membership is now 25 percent Latino.

And Medina's zeroed in on a half-dozen swing states that could prove crucial to the election's outcome: Arizona, California, Colorado, Florida, Nevada and Texas. Medina is also a key organizer of a bus tour and grass-roots campaign dubbed ?Todos A Votar! (Let's All Vote!) that is working alongside leading Latino groups, including NALEO and the National Council of La Raza, to educate and turn out voters.

This election represents a watershed for three reasons, Hispanic organizers said: National Latino groups are working as a coordinated unit for the first time, strict anti-illegal-immigration laws in Arizona and elsewhere have prompted angry backlash in the Latino community and political leaders are vying for the Latino vote - as testified by Romney's and Obama's recent appearances on the Univision cable network.

"For the first time in the history of the Latino vote, the political parties have realized that none of them can really win without Latino participation," said Ben Monterroso, executive director of Mi Familia Vota and Medina's right-hand man. "So the interest has grown."

For Medina, the election is just the midway point in a longer campaign that began a dozen years ago and will continue for another decade and beyond. Medina has a reputation for taking the long view and for thinking strategically. His past labor victories include winning wage increases for janitors and for hospital workers in California.

In his office high atop the SEIU's headquarters in Northwest Washington, D.C., Medina reflected on the lessons he learned picking grapes in the fields alongside his father, who first came to this country as an undocumented worker and became a legal resident.

"I got mistreated like every farm worker," said Medina, whose pale blue dress shirt fits right in at the SEIU, but whose faded jeans and southwestern leather belt recall his years in the fields. "And I learned firsthand what it's like to be discriminated against and mistreated."

For Medina, Election Day is just the beginning of the next campaign. On Nov. 7, he said, he and his allies will launch a push for comprehensive immigration reform and a minimum wage increase. The way Medina sees it, time is on his side: Every month, 50,000 new Latinos turn 18 and become eligible to vote.

"Once our community is mature in terms of age . the Latino vote will be the critical vote for any election," he said.

Clarification

This article has been adjusted to clarify that Mi Familia Vota along with its allied organizations has set out to register 650,000 Latino voters.